Cell differentiation is the central characteristic of morphogenesis which initiates in the embryo, and continues to various degrees throughout the life of an organism in adult tissue repair and regeneration mechanisms. Members of the TGF-.beta. superfamily include subfamilies of highly-related genes that now are suspected to play important roles in cell differentiation and morphogenesis during development and/or during adult life. For example, the Drosophila decapentaplegic gene product (DPP) has been implicated in formation of the dorsal-ventral axis in fruit flies; activins induce mesoderm and anterior structure formation in mammals; Mullerian inhibiting substance (MIS) may be required for male sex development in mammals; growth/differentiation factor-1 (GDF-1) has been implicated in nerve development and maintenance; other morphogenic proteins (BMP-2, -3, -4 and OP-1) induce bone formation.
The development and study of a bone induction model system has identified the developmental cascade of bone differentiation as consisting of chemotaxis of mesenchymal cells, proliferation of these progenitor cells, differentiation of cartilage, ossification and hypertrophy of this cartilaginous tissue, vascular invasion, bone formation, remodeling, and finally, marrow differentiation (Reddi (1981) Collagen Rel. Res. 1:209-206). This bone model system, which is studied in adult mammals, recapitulates the cascade of bone differentiation events that occur in formation of bone in the developing fetus. In other studies, the epithelium of the urinary bladder has been shown to induce new bone formation. Huggins (1931, Arch. Surg. 22:377-408) showed that new bone formation could be induced by surgical transplantation of urinary bladder epithelium onto the parietal fascia. Urist (1965, Science 150:893-899) demonstrated that implantation of demineralized bone segments resulted in endochondral bone formation. The latter study and observation suggested the existence of an osteogenic protein and that bovine diaphyseal bone was a source of enriched preparations of osteogenic protein (Sampath et al., J. Biol. Chem. 265:13198-13205, 1990; Urist, ibid; Reddi et al., Proc. Nat. Aca. Sci. 69:1601-1605, 1972; Sampath et al., Proc. Natl. Acad. Sci. 80:6591-6595, 1983). Proteins capable of inducing endochondral bone formation in mammals when implanted in association with a matrix now have been identified in a number of different mammalian species, as have the genes encoding these proteins, (see, for example, U.S. Pat. No. 4,968,590; U.S. Ser. No. 315,342 filed Feb. 23, 1989; and U.S. Ser. No. 599,543, filed Oct. 18, 1990). Human OP-1 DNA has been cloned from various cDNA and genomic libraries using a consensus probe (Ozkaynak et al., EMBO J. 9:2085-2093, 1990). Purified human recombinant OP-1, expressed in mammalian cells, has been shown to induce new bone formation in vivo. Like other members of the TGF-.beta. superfamily, OP-1 is produced as a precursor, glycosylated, processed and secreted as a mature dimer. Mature OP-1 is cleaved at a maturation site following a sequence with the pattern of RXXR (Panganiban et al., Mol. Cell. Biol. 10:2669-2677, 1990).
The degree of morphogenesis in adult tissue varies among different tissues and depends on, among other factors, the degree of cell turnover in a given tissue. On this basis, tissues can be divided into three broad categories: 1) tissues with static cell populations such as nerve and skeletal muscle where there is little or no cell division and most of the cells formed during development persist throughout adult life and, therefore, possess little or no ability for normal regeneration after injury; 2) tissues containing conditionally renewing populations such as liver where there is generally little cell division but, in response to an appropriate stimulus or injury, cells can divide to produce daughters of the same differentiated cell type; and 3) tissues with permanently renewing populations including blood, bone, testes, and stratified squamous epithelia which are characterized by rapid and continuous cell turnover in the adult. Here, the terminally differentiated cells have a short life span and are replaced through proliferation of a distinct subpopulation of cells, known as stem or progenitor cells.
It is an object of this invention to provide a method of screening compounds which, when administered to a given tissue from a given organism, cause an alteration in the level of morphogenic protein ("morphogen") produced by the tissue. Such compounds, when administered systemically, will result in altered systemic or local levels of morphogenic activity. This morphogenic activity includes the ability to induce proliferation and sequential differentiation of progenitor cells, and the ability to support and maintain the differentiated phenotype or sequence of phenotypes through the progression of events that results in the formation of normal adult tissue (including organ regeneration). Thus, broadly, the invention provides a key to development of additional modalities of therapies involving modulation of morphogenic protein production in animals or adult mammals, e.g., humans, and consequent correction of conditions involving pathologic alteration of the balance of tissue cell turnover. Another object of the invention is to provide methodologies for identifying or selecting a combination of compound(s) which may increase a progenitor cell population in a mammal, stimulate progenitor cells to differentiate in vivo or in vitro, maintain the differentiated phenotype or sequence of phenotypes of a tissue, induce tissue-specific growth in vivo, or replace diseased or damaged tissues or organs in vivo. Another object of the invention is to determine the tissue(s) or organ(s) of origin of a given morphogen. Another object of the invention is to determine the specific cell type(s) within the tissue(s) or organ(s) of origin, or cell line(s) derived from the tissue(s), or organ(s) of origin, that is responsible for the synthesis and production of a given morphogen. These and other objects and features of the invention will be apparent from the description, drawing, and claims which follow.